Some children born to alcoholic mothers suffer an array of mental and motor deficits collectively known as "fetal alcohol effects." Using animal models, researchers are uncovering the fetal defects that may underlie such debilitations.

When pregnant rats are fed ethanol-loaded diets, their fetuses undergo abnormal brain development, according to a report in the Sept. 19 SCIENCE. During normal brain development, a delicate schedule of neuron proliferation and migration ensures the systematic construction of extremely organized brain structures such as the six-layered cerebral cortex. In the study, prenatal exposure to high ethanol levels seemed to upset this schedule in at least three ways.

First, the period of neuron generation started one day later and lasted two days longer in rat fetuses exposed to ethanol, compared with unexposed fetuses. The pregnant rats were fed an amount of ethanol equivalent to what a woman would consume if she drank more than a gallon of beer every day during her pregnancy.

Second, the number of cells generated on particular dayd to the gestation period differed in the exposed and unexposed groups, although the total numbers of neurons were comparable. On most days, fewer cortical neurons proliferated in the ethanol-exposed fetuses than in fetuses from the control group. There was, however, "an anomalous late surge in the generation of neurons in rats exposed to ethanol," reports Michael Miller of the University of Medicine and Dentistry of New Jersey in Piscataway.

Finally, the distribution of neurons generated during this late surge was highly abnormal. Cortical neurons arise in a zone around a fluid-filled cavity called the ventricle, and migrate from there to their specific cortical detinations. Miller observed that many of these late-surge neurons migrated to the "wrong" place.

What does a rat study say about the effects of alcohol on human fetuses? Miller says it suggests that similar developmental defects in the brains of human fetuses probably underlie the symptoms in children with fetal alcohol effects. Researchers agree that heavy drinking should be avoided during pregnancy, when fetuses have precise developmental schedules to stick to. But "precise safe levels cannot be extrapolated from animal research," comments Lyn Weiner, director of the Fetal Alcohol Education Program at the Boston University School of Medicine. Caution is advised, says Weiner, "but the danger of small amounts of alcohol has been exaggerated."

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